Sea Surface Emissivity Model

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ABSTRACT

Recent work to determine the sea water dielectric coefficient was based on laboratory measurements of sea water samples from ifferent parts of the ocean. Although these measurements should render good understanding of the emission from a calm ocean surface, their accuracy in providing values of the ocean still needed to be examined. Our present investigation of the specular sea emission seen from space provides field verification of the sea water specular emissivity over broader regions of the oceans. We investigate and adjust two ocean dielectric models using well calibrated radiometer data, paying particular attention to reducing the frequency dependence of the model and the overall bias of the estimated brightness. In addition, we evaluate the performance of several models for their dependence on salinity and sea temperature.

For this purpose, satellite-based radiometric measurements from the TOPEX/Poseidon project comprising four and a half years are employed together with near-coincident radiosonde profiles from fifteen (15) stations around the world’s oceans and TOPEX altimeter measurements for filtering of low wind conditions. The radiosonde profiles are used to compute the upwelling and downwelling emission and the opacity of the atmosphere. The radiative transfer equation is applied to the radiosonde profiles in order to account for atmospheric effects in the modeled brightness temperature. The dielectric properties of sea water are found from the modified Debye equation using salinity and sea surface temperature data from NODC ocean depth-profiles. The ocean complex permittivity model developed by Klein and Swift and, more recently, by Ellison is tested and revised.

The modified models, ModE and ModKS, exhibit significant improvements in the estimate of TB. Of the two modified models, ModE exhibits superior overall performance, including the lowest bias at both frequencies, which is a very important attribute indicative of the accuracy of the model. Its frequency dependence was decreased to 0.30K, which will allow for more reliable extrapolation to higher frequencies. In addition, ModE has the lowest dependence on sea surface temperature and the lowest RMS difference for both 18GHz and 37GHz. Consequently, this is the model that we recommend for future remote sensing applications involving microwave emissions from the ocean emissivity of the ocean. The average error in the modified emissivity model, over the range 18-40 GHz, is found to be 0.0037, which in terms of brightness temperatures, translates into a model error of approximately 1K.